Process for the continuous preparation of water-absorbent polymers

1. A process for the preparation of water-absorbent polymer particles, comprising the process steps of

preparing an aqueous monomer solution comprising at least partially neutralized, monoethylenically unsaturated monomers bearing carboxylic acid groups (α1) and at least one crosslinker (α3);

adding a polymerization initiator or a at least one component of a polymerization initiator system that comprises two or more components to the aqueous monomer solution;

decreasing the oxygen content of the aqueous monomer solution;

charging the aqueous monomer solution into a polymerization reactor;

polymerizing the monomers in the aqueous monomer solution in the polymerization reactor to form a polymer gel;

discharging the polymer gel out of the polymerization reactor and comminuting the polymer gel thereby obtaining polymer gel particles;

drying the polymer gel particles;

grinding the dried polymer gel particles thereby obtaining particulate water-absorbent polymer particles;

sizing the grinded water-absorbent polymer particles; and

step (xi) crosslinking the surface of the grinded and sized water-absorbent polymer particles;

wherein process step (xi) comprises the steps of:

(x1a) mixing the water-absorbent polymer particles with an aqueous crosslinker solution; and

(x2a) heat-treating the mixture obtained in process step (x1a) in a horizontally operated mixing device,

wherein the horizontally operated mixing device is hydraulically driven.

2. The process according to claim 1 , wherein mixture obtained in process step (x1a) is heat-treated at a temperature in the range from 150 to 250° C. in the horizontally operated mixing device.

3. The process according to claim 1 , wherein the horizontally operated mixing device comprises at least one rotating shaft and a hydraulic motor that converts hydraulic pressure and flow into torque and angular displacement, thereby forcing the shaft into rotation.

4. The process according to claim 3 , wherein at least two paddles are provided on the rotating shafts.

5. The process according to claim 3 , wherein an oil is used as the hydraulic liquid used in the hydraulic motor.

6. The process according to claim 1 , wherein the average residence time of the water-absorbent polymer particles in the horizontally operated mixing device is in the range from 5 to 500 minutes.

7. The process according to claim 3 , wherein the rotation speed of the shaft in the horizontally operated mixing device is in the range from 5 to 25 rpm.